At the time they thought it would weaken the Russians, but obviously they never anticipated a Stalin coming onto the scene. I suppose it does loosely fit the definition of a conspiracy. More like political sabotage.

I agree.

However what about the Hitler/Stalin alliance regarding the invasion and division of Poland?

Further after Hitler's betrayal of Stalin why did Hitler believe he could invade and conquer Russia overnight? This is not what happened, in fact far from it. Germany's humiliation in Russia was one of the turning points in the war. Many commentators have put this down to pure madness on Hitler's part.

But is there more to it than that? Did Hitler have, or believed he had, inside support in the Soviet Union that would swing things in his favour? This is very much how the man Hitler thought. He had many schemes on the boil.